In a crude oil refinery, generally the oil is pumped from a storage tank to a crude unit for processing. The crude unit cleans the oil through water washing in a desalter and then splits the oil into fractions in an atmospheric distillation tower. These fractions are pumped to various processing units downstream of the crude unit (e.g., coker, catalytic cracker, hydrotreater etc.). Though corrosion and corrosion byproduct deposition (the latter sometimes referred to herein as fouling) occur in many areas of a crude unit, the most severe corrosion and fouling typically take place in the overhead condensing system of the atmospheric distillation tower.
Refinery crude unit processing has becoming increasingly difficult in recent years and is predicted to become even more challenging and complex for several reasons. For example, significant increases in crude oil prices have caused refiners to aggressively pursue “opportunity” or “challenging” crudes that are obtainable at discounted prices. The lower price is linked to a crude property such as high acid or high solids content that makes it less desirable than the light, sweet benchmark crudes.
Refiners switch crude slates more frequently than in the past due to minimum on-hand crude oil inventory combined with increased crude oil variety. A crude slate switch typically upsets the steady state condition of a crude unit for up to several hours. Generally, about eighty percent of the corrosion and fouling occurs during these switches or disruptions, which normally last about twenty percent of the time. If fouling and corrosion issues are severe enough, the refiner will discontinue processing the crude oil or blend of crudes causing the problem. However, these challenging crudes are available to the refiner at a discount thus making them more profitable. Discontinuing such problematic crudes is accordingly not a very popular option.
In efforts to reduce corrosion, a crude unit may be serviced two or three dines per week, or in sonic cases daily. Daily service at best provides a snap shot view of a dynamic crude unit system. Crude type and/or raw crude storage tanks are switched several times per week, sometimes daily. The contents of each tank are different from the others, so each switch causes a change of feed quality to the crude unit, many times upsetting the steady state status and causing disruptions in the system. Preheating, desalting, and distilling operations shift with the new crude, sending products and/or effluent water sources off specification. Many adjustments over several hours (in some cases days) normally take place to return the crude unit to steady state operation.
The most common current industry practice to control such disruptions and optimize crude unit operation is to provide enough manpower and man-hours. For instance, each crude unit may have an operating crew from three to ten people, depending on size and complexity of the unit. This crew may spend their day gathering various samples for wet chemistry lab testing, and measuring and making adjustments for temperature and flow to keep the unit running within specification. Such practice is typically geared towards keeping the unit operating properly with respect to fractionation quality cut points and end points, with minimal attention being paid to a specialty chemical corrosion control program. If a disruption is severe, changes may be made to the process chemicals and/or changes in levels, flows, or temperatures may be recommended around the crude unit to keep the dynamic system in as optimum a condition as possible.
Attempts to compensate for periodic or sometimes prolonged lack of human involvement include installing online pH meters on atmospheric distillation towers overhead accumulator water boots; however, due to a high rate of fouling of the pH sensor only a small percentage of these meters operate correctly for any length of time. Online instrumentation, such as pH meters, requires routine maintenance and calibration. Moreover, online pH merely tracks the pH and sends an alarm to the operator when the pH is outside the control limits. Often, poorly calibrated and/or fouled pH meters cause frequent alarms. This frequency tends to minimize the effectiveness of the alarm system.
Due to the lack of industry success with online pH metering and other monitoring efforts refiners have not pursued more exotic and effective online instrumentation for process chemical programs. There thus exists an ongoing need for more sophisticated and effective online and/or automatic methods for monitoring parameters and reducing corrosion in crude units.